Information printed with bleed through or penetrating ink is a document security feature that is common in checks and other monetary instruments. The bleed through effect makes alteration of the document much more difficult. Verification of the security feature is possible without additional tools.
Most commonly, bleed through inks consist of a black colorant that stays on the print side of the media and a magenta colorant that penetrates the media and creates a visible stain on the non-print side of the media. For printing of fixed data, inks can be printed specifically by dry or wet offset by screen-, flexo- and gravure-printing or by letterpress.
For variable data such as check numbers, this security feature is most commonly applied via impact printing. The inks used in impact printing are often oil based so that the drying time is extended, thus allowing the dye to penetrate as shown in European Patent No. 0 835 292 B1. Characters or linear barcodes are imprinted using coding wheels such as the impact numbering machines offered by the company Atlantic Zeiser.
A disadvantage of the impact printing approach is that there is limited flexibility to design the printed information. It is limited to a fixed number of digits of numbers and some one-dimensional barcodes of fixed length.
Two-dimensional barcodes and graphics cannot be printed in this manner. Therefore, it is desirable to have a digital, non-impact way of delivering this security feature such as continuous inkjet printing. It can be difficult, however, to formulate a continuous inkjet ink that has the necessary penetrating and drying properties while maintaining good drop generation and recirculation properties and image quality. It is particularly difficult to generate a two component ink wherein one colorant stays on the print side of the media while the other penetrates to the non-print side.
An inkjet printing ink that does not need to have unusual penetrating properties, but that still forms an image on the non-print side of the media would therefore be desirable.